Baseball and softball bat manufacturers are continually attempting to develop ball bats that exhibit increased durability and improved performance characteristics. Ball bats typically include a handle, a barrel, and a tapered section joining the handle to the barrel. The outer shell of these bats is generally formed from aluminum or another suitable metal, and/or one or more composite materials.
Barrel construction is particularly important in modern bat design. Barrels having a single-wall construction, and more recently, a multi-wall construction, have been developed. Modern ball bats typically include a hollow interior, such that the bats are relatively lightweight and allow a ball player to generate substantial “bat speed” or “swing speed.”
Single-wall bats generally include a single tubular spring in the barrel section. Multi-wall barrels typically include two or more tubular springs, or similar structures, that may be of the same or different material composition, in the barrel section. The tubular springs in these multi-wall bats are typically either in contact with one another, such that they form friction joints, are bonded to one another with weld or bonding adhesive, or are separated from one another forming frictionless joints. If the tubular springs are bonded using a structural adhesive, or other structural bonding material, the barrel is essentially a single-wall construction. U.S. Pat. No. 5,364,095, the disclosure of which is herein incorporated by reference, describes a variety of bats having multi-walled barrel constructions.
It is generally desirable to have a bat barrel that is durable, while also exhibiting optimal performance characteristics. Hollow bats typically exhibit a phenomenon known as “trampoline effect,” which essentially refers to the rebound velocity of a ball leaving the bat barrel as a result of dynamic coupling between the bat and the ball. It is desirable to construct a ball bat having a high “trampoline effect,” so that the bat may provide a high rebound velocity to a pitched ball upon contact.
The “trampoline effect” is a direct result of the matching of fundamental frequencies between the bat and the ball (dynamic coupling), and the resulting compression and strain recovery of the bat barrel. During this process of barrel compression and decompression, energy is transferred to the ball resulting in an effective coefficient of restitution (COR) of the ball, which is the ratio of the post impact ball velocity to the incident ball velocity (COR=Vpost impact/Vincident). In other words, in general, the COR of the ball improves as the “trampoline effect” increases.
Multi-walled bats were developed in an effort to increase the amount of acceptable barrel deflection beyond that which is possible in typical single-wall and solid wood designs. These multi-walled constructions generally provide added barrel deflection, without increasing stresses beyond the material limits' of the barrel materials. Accordingly, multi-wall barrels are typically more efficient at transferring energy back to the ball. In general, multi-walled bats accomplish higher performance by lowering the barrel stiffness through decoupling of the shear interfaces between the barrel layers. The lower barrel stiffness decreases the highly inefficient ball deformation and increases barrel deformation. Barrel deformation is more efficient in returning the impact energy to the ball, thus resulting in improved performance.
An example of a multi-wall ball bat 100 is illustrated in FIG. 1. The barrel 102 of the ball bat 100 includes an inner wall 104 separated from an outer wall 106 by an interface shear control zone (“ISCZ”) 108 or layer, such as an elastomeric layer, a friction joint, a bond-inhibiting layer, or another suitable shear-controlling zone or layer. Each of the inner and outer walls 104, 106 typically includes one or more plies 110 of one or more fiber-reinforced composite materials. Additionally, or alternatively, one or both of the inner and outer walls 104, 106 may include a metallic material, such as aluminum.
One way that a multi-wall bat differs from a single-wall bat is that there is no shear energy transfer through the ISCZ(s) in the multi-wall barrel, i.e., through the region(s) between the barrel walls that de-couple the shear interface between those walls. As a result of strain energy equilibrium, this shear energy, which creates shear deformation in a single-wall barrel, is converted into bending energy in a multi-wall barrel. And since bending deformation is more efficient in transferring energy than is shear deformation, the walls of a multi-wall bat typically exhibit a lower strain energy loss than does a single wall design. Thus, multi-wall barrels are generally preferred over single-wall barrels for producing efficient bat-ball collision dynamics, or more efficient dynamic coupling “trampoline effect.”
To illustrate, FIG. 2 shows a graphical comparison of the relative performance characteristics of a typical wood bat barrel, a typical single-wall bat barrel, and a typical double-wall bat barrel. As FIG. 2 illustrates, double-wall bats generally perform better along the length of the barrel than do single-wall bats and wood bats. While double-wall bats have generally produced improved results along the barrel length, these results still decrease as impact occurs away from the barrel's “sweet spot.”
The sweet spot is the impact location in the barrel where the transfer of energy from the bat to the ball is maximal, while the transfer of energy to a player's hands is minimal. The sweet spot is generally located at the intersection of the bat's center of percussion (COP), and the superposition of the first three axial fundamental modes of vibration. This location, which is typically about 4 to 8 inches from the free end of the barrel (it is shown at 6 inches from the free end of the barrel in FIG. 2, by way of example only), does not move when the bat is vibrating in its fundamental bending modes. As a result, when a ball impacts the sweet spot, the bat vibration energy loss is minimal, and a player swinging the bat does not feel vibration.
The barrel regions between the sweet spot and the free end of the barrel, and between the sweet spot and the tapered section (and beyond) of the bat, in particular, do not exhibit the optimal performance characteristics that occur at the sweet spot, due to energy loss resulting from vibration and rotational inertia effects. Indeed, as shown in FIG. 2, in a typical ball bat, the barrel performance decreases considerably as the impact location moves away from the sweet spot. As a result, a player is required to make very precise contact with a pitched ball, which is generally very challenging to do, to achieve optimal results and to avoid stinging bat vibration. Thus, a need exists for a ball bat that exhibits improved performance at regions of the ball bat away from the sweet spot. Additionally, a need exists for an improved single-wall bat that exhibits improved performance characteristics.